Desiccative aging

Most adsorbers operate on a fixed drying cycle time and, frequently, the cycle lime is set for the worst conditions. However, the adsorbent capacity is not a fixed value it declines with usage. For the first few months of operation, a new desiccant has a very high capacity for water removal. If a moisture analyzer is used on the effluent gas, a much longer initial drying cycle can be achieved. As the desiccant ages, the cycle time will be automatically shortened. This will save regeneration fuel costs and improve the desiccant life. 

The fact that property differences between desiccated and soaked samples exist reinforce the concept that physical modifications are introduced into the polymer network by the moisture-temperature ageing. While the exact nature of this change... 

Glass-ionomer cements become less susceptible to desiccation as they age, because a greater proportion of the water in older cements has become tightly bound . Early contact with moisture is also damaging, and this problem is overcome clinically to some extent by using some sort of protection such as clear nail varnish to seal the cement during its early life (Wilson McLean, 1988). However, this does not give perfect results, and as yet there is no ideal barrier material for this purpose (Earl, Hume Mount, 1985). 

The main technique that has been used for the measurement of opacity has been to prepare a standard disc of AB cement 1-0 mm thick and aged for 24 hours at 37 °C. This disc, contained in a small trough of water to prevent desiccation, is placed in a reflectometer on a black background. It is then illuminated with diffuse light and the amount of light reflected from it, is measured. The disc is then placed on a white background of 70% reflectivity, and the new amount of reflected light, measured. The contrast ratio 7 o/ o is defined as the Cp., opacity (Crisp et al., 1979). 

But certain conditions such as freezing or desiccation (removal of water) can greatly reduce the activity of these enzymes. This means that mummies such as Iceman, despite its great age, may contain some quantity of DNA in good enough shape to determine its sequence. There are other enemies of ancient DNA, though, including radiation, which breaks apart the strands, and chemical reactions such as oxidation and... 

All samples were dried in a constant stream of air for several hours and then dried for at least four days under vacuum at room temperature. After drying, the films were aged in desiccators at room temperature for at least two weeks. This allows the PCL crystallinity to approach its equilibrium value (6). Throughout this chapter the blends are designated by the weight percent of the polymer blended with polycaprolac-tone. Thus 25% PVC is a 25/75 blend of PVC/PCL. 

Artificial Aging by Heat. The procedure developed earlier (11) was followed. Fabric pieces (15 X 15 cm) were placed on racks covered with a Fiberglas screen (7- X 3-cm mesh) in a forced convection oven preheated to 150 °C. The screen was used to prevent any enhanced degradation that might be caused by direct contact with the metal rack. The control silk fabric was heated for up to 4 days in 6-h increments, and then immediately placed in a desiccator that contained silica gel to keep the silk dry while cooling. 

Following soaking, samples were rinsed with deionized water, squeezed gently, and allowed to air dry overnight on glass-fiber screens. All samples were stored in desiccators over silica gel prior to accelerated aging. 

Accelerated Aging. Samples were removed from the desiccators and placed on glass-fiber screens in a forced-draft oven at the required temperature. Upon removal from the oven, samples were collected in a desiccator and then were transferred to a conditioned laboratory for testing. 

The stability of composition-graded films seems to be good because the adhesion value and chemical analysis data obtained by XPS do not show any significant changes after aging 2 months in a desiccator. 

The dehumidification efficiency of silica desiccants very depends on the synthesis and aging conditions such as pH[l,2]. But, there seems to be a limit to improve the dehumidification efficiency by using pure silica desiccants. 

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